The present invention relates to a low-profile ultrasonic sheet feeder utilizing ultrasonic vibrations, a low-profile ultrasonic motor, and a method of driving the same, which are intended to be applied to office automation equipment, e.g., facsimile apparatuses and printers.
In order to realize low-profile sheet feeding mechanisms used for office automation equipment and the like, ultrasonic motors using flat vibrators have been enthusiastically studied.
An ultrasonic feeder using a vertical flexural multiple mode flat vibrator has already been proposed in Japanese Patent Application Nos. 62-126634 and 62-126636 (Japanese patent Application Laid-Open Nos. 63-290176 and 63-294270). In addition, the design, trial manufacture, and evaluation results about the ultrasonic sheet feeder using the flat vibrator have been reported in the Transactions of the Institute of Electronics, Information and Communication Engineers in the Spring Convention of 1988, "A-224" and "A-225".
Of these conventional devices, a structure of a vertical flexural multiple mode flat vibrator is shown in FIGS. 19(a) and 19(b). FIG. 19(a) is a plan view of the vibrator. FIG. 19(b) shows a front view of the vibrator and a circuit for driving the vibrator. Referring to FIGS. 19(a) and 19(b), reference numeral 100 denotes a rectangular metal plate; and 111, 112, and 113, piezoelectric ceramic plates. The piezoelectric ceramic plate 111 serves to mainly excite a vertical fundamental resonance mode. The piezoelectric ceramic plates 112 and 113 serve to mainly excite flexural vibrations. Reference numeral 120 denotes an oscillator; 121, a phase shifter; and 122 and 123, amplifiers.
FIGS. 20(a) and 20(b) show the vibration mode of this vertical flexural multiple piezoelectric vibrator. FIG. 20(a) is a plan view of the vibrator. FIG. 20(b) is a front view of the vibrator. The broken line in FIG. 20(b) indicates the vibration displacement distribution of vertical fundamental resonance. The broken lines in FIG. 20(a) indicate vibration nodes of the flexural vibration mode (B.sub.2--2 mode). A resonance frequency f.sub.B22 of the flexural vibration B.sub.2--2 mode is dependent on a width W.sub.0 and a plate thickness T.sub.0 and is inversely proportional to the width W.sub.0. In addition, a resonance frequency f.sub.L1 of vertical vibrations is dependent on a length L.sub.0 and is inversely proportional to the length L.sub.0. Therefore, by properly setting the width W.sub.0 and the length L.sub.0, f.sub.L1 =f.sub.B22 can be realized. Then, both the vertical vibration fundamental primary mode and the flexural vibration B.sub.2--2 mode can be resonated for the first time.
By setting a phase difference of .+-.90.degree. between vertical vibrations and flexural vibrations, a large-amplitude elliptic vibration can be produced near a point P or Q in FIG. 20. A phase difference can be easily set between vertical vibrations and flexural vibrations by causing the phase shifter 121 to set a phase difference between AC voltages respectively applied to the piezoelectric ceramic plate 111 and the piezoelectric ceramic plates 112 and 113.
FIG. 21 shows an ultrasonic sheet feeder using the vertical flexural double mode vibrator shown in FIG. 19. Referring to FIG. 21, reference numeral 114 denotes a plate consisting of a flexible material such as rubber; 115, a hard metal base; 116, a roller; and 117, a paper sheet. In addition, an arrow F.sub.0 indicates a force which urges the roller 116 against the flat vibrator. Vibration displacement based on the flexural vibrations of the flat vibrator occurs in the Z-axis direction, whereas vibration displacement of the vertical vibrator occurs in the X-axis direction. If a paper sheet is placed on the flat vibrator, and the roller 116 is pressed on the sheet at the point P in FIG. 20, an elliptic vibration composed of vibration displacements in the X- and Z-axis directions is produced at the point P at which the roller 116 is in contact with the paper sheet. As a result, the paper sheet 117 can be moved in the positive or negative X-axis direction through a frictional force generated between the rectangular metal plate 100 of the flat vibrator and the paper sheet 117. Whether to move the paper sheet 117 in the positive or negative X-axis direction can be easily selected by properly setting the phase difference between voltages to be applied to the piezoelectric ceramic plate 111 and the piezoelectric ceramic plates 112 and 113.
The following problem, however, is posed in the ultrasonic sheet feeder using such a conventional vertical flexural double mode vibrator. If, for example, the sheet feeder uses a general vibrator having a thickness T.sub.0 of 2 mm, a length L.sub.0 of 70 mm, and a width W.sub.0 of 15 mm, a large speed of about 55 mm/s can be relatively easily obtained as the moving speed of a paper sheet. However, only a thrust of 300 gf can be obtained at best with a contact force F of 1 kgf. Although, such a feeder is suitable for a light load, e.g., a card feeder, it is far from satisfactory as a sheet feeder for a facsimile apparatus requiring a high thrust (e.g., 1 kgf or more).
This is because if the contact force F.sub.0 is increased to obtain a high thrust, flexural vibrations are suppressed due to the use of the flat vibrator. In addition, a silicone rubber sheet 114 such as the plate 111 is used so as not to suppress flexural vibrations. However, if the ultrasonic sheet feeder is realized with a resonance frequency of about 30 kHz, this silicone rubber sheet 114 is required to have a thickness of at least about 5 mm. That is, the total thickness of this sheet feeder including the roller 116 becomes 20 mm or more. Therefore, the silicone rubber sheet 114 interferes with the development of a low-profile sheet feeder.
In order to increase the feed speed, resonance driving must be performed while the vertical and flexural resonance frequencies of the double mode flat vibrator are set to coincide with each other. However, frequency adjustment for this operation is very complicated, resulting in difficulties in the manufacturing process.